The present invention relates to a method for forming high efficiency light-emitting diode (LED), and more particularly, to a method for fabricating high efficiency light-emitting diode adhered to a transparent substrate, such as the material of glass.
A light-emitting diode (LED) is an important optoelectronic device which is widely used in various products such as electronic devices, advertising signs and electrical appliance to indicate the related messages or to sharpen the beauty of a product. Additionally, the utilization of the LEDs has been gradually expanding to all kinds of application. The high brightness becomes a significant index level of the superior quality LEDs besides true color and duration of the LEDs.
Referring to FIG. 1A, GaAs is used as a semiconductor substrate for the LEDs. To begin with, a double heterostructure 110 is formed on a GaAs substrate 10 wherein the double heterostructure 110 comprises a N-type cladding layer of AlGaInP 11, an active layer of AlGaInP 12 and a P-type cladding layer of AlGaInP 13, and a transparent layer 14 on the P-type cladding layer of AlGaInP 13. Since the energy band gap of the GaAs substrate 10 is less than that of the active layer of AlGaInP 12, the energy band gap of the GaAs substrate 10 lies in the range of the visible light. Consequently, the specific wavelengths of visible light, induced from the double heterostructure 110, may be absorbed by the GaAs substrate 10 which results in brightness degradation of the LED.
Referring to FIG. 1B, GaP is used as a semiconductor substrate for the LEDs. A double heterostructure 110 is formed on a GaP substrate 15 wherein the double heterostructure 110 comprises a N-type cladding layer of AlGaInP 11, an active layer of AlGaInP 12 and a P-type cladding layer of AlGaInP 13, and a transparent layer 14 on the P-type cladding layer of AlGaInP 13. Since the GaP may induce a light-emitting absorption in the range of spectrum (yellow and green), the conventional material is no more than applied to the use of some particular wavelength of LED.
Referring to FIG. 2, a Bragg reflector is applied on a GaAs semiconductor substrate. A Bragg reflector 21 formed on the GaAs substrate 10 and the double heterostructure 110 is formed on a GaAs substrate 20 wherein the double heterostructure 110 comprises an N-type cladding layer of AlGaInP 11, an active layer of AlGaInP 12 and a P-type cladding layer of AlGaInP 13, and a transparent layer 14. The material of the Bragg reflector 21 is GaAs/AIAs having a coefficient 3.4 Although the Bragg reflector 21 can partially eliminate the absorption of the light emitted from the double heterostructure 110 to prevent the light-emitting from being absorbed by the GaAs substrate 10. Unfortunately, the Bragg reflector 21 may increase the complexity of the process and thus largely spend a great of epitaxy time during manufacture process. Also, the sufficient thickness of the Bragg reflector 21 is needed, even so, the Bragg reflector 21 can not completely solve the light-emitting absorption problem by the GaAs substrate 10.
Consequently, a conventional method for fabricating a LED according to the above involves some disadvantages as follows: (a) the visible light, emitted from the double heterostructure, will be absorbed by the GaAs substrate and result in a great deal of the light-emitting efficiency reduction; (b) a conventional material, GaP compound material, for a transparent layer may incur the absorption of some specific wavelength band (yellow or green) so that the transparent layer is only used in the particular range of the spectrum; and (c) the Bragg reflector may obviouly increase the manufacturing time and the production cost, especially, when multiple Bragg reflector layers are used.
The primary object of the present invention is to provide a new method of fabricating high efficiency light-emitting diode (LED) adhered to a transparent substrate, preferably a glass adhered thereon by heat and to overcome the drawback of the lower light-emitting efficiency regarding the conventional LED.
In the preferred embodiment of the present invention, to begin with, providing a semiconductor substrate is followed by forming an etching stopper, a first type ohmic contact layer, a double heterostructure and then a second ohmic contact layer. The double heterostructure comprises a N-type cladding layer of AlGaInP, an active layer of AlGaInP and a P-type cladding layer of AlGaInP. Afterwards, a transparent substrate, such as preferably glass, which is visible in the range of visible light, is adhered to the second ohmic contact layer and then the GaAs substrate is removed away. The first type cladding layer and the undoped active layer are etched, wherein the second type ohmic contact layer is used as the etching stopper. The first electrode is formed on the first type ohmic contact layer and the second electrode is formed on the second type ohmic contact layer, respectively.
As a result, the present invention utilizes some features, for example, high transmittance and lower absorptivity, of the glass adhered to the second ohmic contact layer to visible light for increasing luminous intensity of the LED. Taking advantage of glass in strength and machining, the surface of the glass is easily roughed so that the total reflection inside the glass is entirely blocked while the photons are transmitted from the LED therethrough. Moreover, the absorption of the GaAs substrate for the visible light is significantly improved by appropriately decreasing the use of the GaAs material.